In general, according to Fresnel s law by which reflection of light emitted at an interface between materials with different refractive indices increases with an increase in the difference between the refractive indices, reflection of light emitted at the interface due to a difference in refractive index between a photo device including a material with a high refractive index and the air may be directly related with the performance of the photo device, and minimizing reflection of light may be necessary to fabricate a photo device with good performance. Accordingly, development of a technique for minimizing reflection of light emitted between the photo device and the air using a simple method for a short amount of time at low cost has progressed.
For example, in order to reduce reflection of light in photo devices, such as solar cells, photodetectors, emitting diodes, and transparent glasses to improve the luminous efficiency and performance of the photo devices, an anti-reflective coating (ARC) method and a surface texturing method have typically been used as anti-reflection methods.
The ARC method includes depositing a material having a lower refractive index than a semiconductor material on the semiconductor material to reduce a sharp difference in refractive index between the semiconductor material and the air and lessen reflection of light. In this case, a single or multiple anti-reflective layer may be deposited.
Although the refractive index and optical thickness of a coating material may be controlled using the ARC method to minimize a reflectance in a specific wavelength range, selection of a coating material used as an anti-reflective layer is limited according to the type of a semiconductor material. Also, a reflectance and a wavelength range may depend on the electrical and thermal properties of the coating material. Furthermore, it is difficult to reduce the reflectance over a wide wavelength range and a wide incident angle.
The surface texturing method includes forming a regular or irregular structure or bend on the surface of a semiconductor material using a physical etching process or chemical etching process to reduce reflection of light on the surface of the semiconductor material.
The physical etching process used for the surface texturing method may be, for example, a plasma etching process, a photolithography process, or a mechanical scribing process. Although these processes do not cause disparity in etch rate according to a crystallization direction of a semiconductor substrate and enable a reduction in reflectance, the processes are complicated, take long amounts of time, preclude mass production, and require high-priced vacuum apparatuses and additional equipment. Accordingly, the above-described physical etching processes may be commercially inaccessible.
By comparison, in the chemical etching process used for the surface texturing method, a surface shape and etch rate may be varied according to the crystallization direction of the semiconductor substrate, the type of a component, a composition ratio, and a doping type and forming a fine structure may be difficult. However, as compared with the physical etching process, the chemical etching process may require simpler process steps and shorter process times, enable low-cost mass production of photo devices, and facilitate process control, so the chemical etching process has been under study for the purpose of texturing a semiconductor surface.
In recent years, a vast amount of research has been conducted on a method of fabricating a subwavelength structure (SWS), which overcomes the restrictions of the ARC method, obtains a much lower reflectance in a wider wavelength range than a surface texturing method, and exhibits a low reflectance over a wide range of incident angles.
Conventionally, fabrication of an SWS may include forming a subwavelength periodic or non-periodic pattern on a substrate using an electronic beam (e-beam) lithography process or hologram lithography process and performing a physical etching process or chemical etching process using the subwavelength periodic or non-periodic pattern. Alternatively, the fabrication of the SWS may be performed using a nano-imprint process and a lift-off process. However, these conventional methods are uneconomical due to complicated process steps, low productivity, and long process times.
Accordingly, it is absolutely necessary to develop techniques of integrating a photo device with a subwavelength nanostructure for antireflection with high luminous efficiency using a chemical etching process and produce devices with good luminous efficiency and performance in large quantities using simple processes, short process times, low fabrication costs, and easy process control.